A magnetic random access memory (MRAM) has features, such as a high-speed read/write operation of several tens of nanoseconds or less, low power consumption, and non-volatility. Therefore, the MRAM attracts attention as a universal memory having all features of a dynamic random access memory (DRAM), a static random access memory (SRAM), and a flash memory. In the MRAM, one memory element comprises a magnetic tunnel junction (MTJ) element in which information is stored and a selection transistor that selects the specific MTJ element.
The MTJ element has a structure in which a thin insulating film is formed between two ferromagnetic layers. In the MTJ element, a resistance decreases in the case that magnetization directions of two ferromagnetic layers are parallel to each other by a magnetoresistive effect, and the resistance increases in the case that the magnetization directions are antiparallel to each other. The two states are distinguished as “0” and “1” to store the information. At this point, one of the ferromagnetic layers is a reference layer in which the magnetization direction is invariable, and the other ferromagnetic layer is a storage layer in which the magnetization direction is variable.
Spin transfer is used to write “0” and “1” in the MTJ element. The spin transfer is a method in which a current having an electron, in which the magnetization direction is polarized in one direction, flows through the MTJ element to directly rewrite the magnetization direction of the storage layer. The current flowing through the MTJ element to write the information is called a write current. It is necessary to decrease the write current in order to increase a capacity of the MRAM. One of the solving methods is that the magnetization directions of the storage layer and the reference layer of the MRAM element are changed from an in-plane direction to a perpendicular direction.
However, a stray field generated from the reference layer acts on the storage layer when the reference layer has the perpendicular magnetization. Particularly, a large stray field opposite to the magnetization direction of the reference layer acts on an end portion of the storage layer. The stray field acting on the storage layer significantly disturbs coherent magnetization rotation of the storage layer during the spin transfer write, which increases the write current. Additionally, a distribution of the stray field becomes uneven with respect to the storage layer, which degrades a retention characteristic of the storage layer.
The stray field from an adjacent MTJ element also acts on the storage layer. For example, a cell size (area) equal to that of the DRAM is required when a Gbit-order MRAM is made at the same cost as the DRAM. That is, assuming that “F” is a minimum size of “lithography, the Gbit-order MRAM has cell sizes of 8F2 to 6F2. At this point, the MTJ element is produced with a cell size of F2, and a distance to the adjacent MTJ element becomes about F. In the Gbit-order MRAM, F is as extremely small as about 45 nm. Because the distance between the elements is extremely small, the stray field from the reference layer acts on the adjacent element. The stray field from the adjacent element acts on the storage layer, and distribution of the stray field is uneven, which results in the increase in write current and the degradation of the retention characteristic. Additionally, because the influence of the stray field from the adjacent element depends on the element, a variation in write current increases among the elements.